1. Field of The Invention
The present invention relates to an improved process for processing thermoplastic polymers in various types of fabrication equipment. More particularly, the present invention provides a processing aid comprising, at least, a processing additive and a processing adjuvant. The processing additive is a polymeric material having an hydroxy, alkoxy, epoxy, carboxy or amino group. The processing adjuvant contains at least two monofunctional groups or at least one multifunctional group wherein at least one functional group provides preferential adsorption over the processing additive for the fabrication surface and wherein at least one other functional group is capable of bonding with the processing additive.
2. Prior Art
The manufacture of thermoplastic parts by melt fabrication processes such as extrusion and molding is generally not possible using neat polymers directly as synthesized. Instead, it is common practice to "formulate" compositions containing a variety of ingredients in relatively small, but critical amounts. These ingredients may be categorized into two main and fairly distinct groups, namely product additives and processing aids.
The product additives, which primarily serve the function of modifying the properties of the fabricated material, include pigments and dyes (colorants), heat stabilizers and antioxidants, light and UV stabilizers, antistatic agents, slip and antiblocking agents, and the like.
The processing aids primarily, if not exclusively, facilitate processing--often to the point that processing would be impossible without them. Foremost among these aids are lubricants, sometimes referred to as release agents, which prevent sticking of the hot molten thermoplastic polymer to fabrication surfaces such as extruder screws, extrusion dies, mill and calender rolls, injection molds, and the like. In addition, lubricants can have many other beneficial functions in the processing of molten thermoplastic polymers. In spite of the often critical importance of these processing aids to the thermoplastic fabrication industry, interface effects in polymer melt flow have received scant attention in the past and little is still known regarding the chemistry and physics of boundary phenomena. In fact, even in the determination of melt viscosity--the principal material parameter of polymer melt flow--the interface effects are usually ignored by the expediency of assuming that the polymer melt velocity always is zero at the solid boundary.
Recent studies of "melt fracture", a flow instability phenomenon occurring at high flow rates during extrusion of thermoplastic polymers, demonstrate the importance of the "micro rheology" at the fabrication surface/polymer melt boundary. Several mechanisms have been proposed for the occurrence of gross melt fracture and there is no general agreement on either the mechanism or the site of initiation of this defect. The melt fracture phenomenon manifests itself by severe surface irregularities in the extrudate as it emerges from the die. For a given polymer/die material combination, flow geometry and processing temperature, the surface defects occur above a critical nominal shear stress. In polyethylene, melt fracture has now persuasively been attributed to a breakdown in the adhesion between the polymer melt and the rigid die surface in the "land" (exit) region of the die (A. V. Ramamurthy, J. Rheology, 30(2), 337-357, 1986 and Advances in Polymer Technology, 6(4), 489-499, 1986). In a further development, it has now also been shown that the introduction of a liquid additive, tailored so as to be strongly bonded to the die surface, yet "compatible" with the polymer melt, allows dramatic increases in flow rates before the surface defects typical of melt fracture are incurred (W. B. Herdle and W. A. Fraser, "Improvements in Blown Film Extrusion of Polyolefins Containing a Novel Processing Aid", Society of Plastics Engineers, Los Angeles, Calif., May 1987).
It is noteworthy that in both cases, the chemistry and the micro rheology at the boundary between the polymer melt and the solid fabrication surface greatly overshadowed the effects of other relevant variables such as the details of the die geometry and of the molecular architecture of the polymer.
The problems of melt fracture during high rate extrusion of polyolefins have in the past been alleviated by the use of fluorocarbon compounds (e.g. duPont: Viton A), fluoroelastomers (e.g. 3M: Dynamar PPA 2231), sulfonated fluorocarbon polymers and fluoroalkyl sulfonates (Mitsui, Japanese Patent No. 59/113058, June 29, 1984); refer also to the article by Rudin, Worm and Blacklock in Plastics Engineering, 63-66, March 1986. These processing aids are effectice in reducing melt fracture in the extrusion of polyolefins, especially linear low density polyethylene, which are particularly prone to melt fracture. The main problems arising in the commercial use of these processing aids are difficulties in dispersion of the processing aid and a tendency for plate-out of decomposed substances on the extruder screw and/or the die lips. The plate-out is often severe, requiring shut-down of the equipment and extensive clean-ups. In addition, fluorocarbon materials are inherently expensive.
Another approach is described in U.S. Pat. No. 4,535,113. This invention discloses organo modified silicone processing aids which effectively reduce melt fracture and have been shown to improve through-put under static power conditions, yet do not have the drawback of plate out. Unfortunately, the performance of these organo modified silicone compounds is often greatly diminished in the presence of certain other thermoplastic additives such as zinc stearate, which commonly are used as mold release agents and as inhibitors for the discoloration of polyolefin products. This interference by conventional additives used in the commercial manufacture of polyolefins is a drawback to their general utility.
A still third approach relies on a change in the extrusion die materials of construction to a metal or alloy showing better wetting and adhesion to molten polyolefins than do standard steel or chrome plated dies ( U.S. Pat. Nos. 4,552,776; 4,552,712 and 4,554,120). Although this "hardware" solution works, the metals showing improvements in alleviating melt fracture are quite soft (e.g. copper alloys), hence their use is limited in manufacturing situations due to the high risk of damage. Besides, new equipment and shut-downs for repair are costly.
The most common reason for using lubricants is to reduce the tendency for a thermoplastic polymer to stick to the hot metal surfaces of dies, molds and rolls. A great variety of chemicals, oils, waxes and soaps have and are being used in different thermoplastic polymers, refer for example to the review in E. W. Flick: "Plastic Additives", Noyes Publications, Park Ridge, N.J. 1986, especially Section XII on lubricants. Another general overview is given by G. Illmann in an article entitled "Waxes As Lubricants In Plastics Processing", SPE Journal, pp. 71-76, 121, 1967. See also U.S. Pat. No. 4.371.476. In addition to securing release of the thermoplastic polymer from a hot metal surface, commercial lubricants must meet a number of other criteria such as freedom from objectionable odors, freedom from making clear plastics hazy or opaque, freedom from exuding to the surface and making it tacky or waxy, etc. In isolated instances, certain lubricants have been observed to contribute to alleviating some of the other processing problems enumerated above. For example, Japanese Patent No. 59/46527, Nov. 13, 1984 notes that certain polycarbonyl compounds alleviate the plate-out on hot rollers of a barium soap stabilizer in PVC; P. L. Shah in a paper entitled "Influence of Shear Dependent Lubricant Characteristics on Melt Rheology of PVC", Soc. Plastics Engineers, Tech Papers, 17, 321-325, 1971, states that stearic acid reduces melt fracture in plasticized PVC; Rudin et al, referred to above, noted that polyolefins could be extruded at lower back pressures and/or higher throughput rates in the presence of fluorocarbon lubricants; Japanese Patent No. 49/15948, Apr. 18, 1974 noted that poly(dimethyl siloxane) increased the output and lowered the power requirements in extrusion of polyethylene; Duska, Gasior and Pomper in a paper entitled "Effects of Grooved Feed Throat on Extruder Performance", Soc. Plastics Engineers, Techn. Papers, 21, 434-438, 1975 reported in a study covering seven different plastics that in each case a maximum lubrication level existed above which the extruder would not feed (screw fouling), even with a grooved barrel section. References to solving other processing problems through the use of lubricants or other processing aids are sparse or non-existent.
In summary, in the prior art, practical solutions to the processing problem of "sticking" have generally been achieved for all of the major thermoplastics. On the other hand, solutions to the problem of melt fracture have so far been achieved only with the concomitant creation of other problems such as dispersion difficulties of the fluorocarbons, plate-out of decomposition products, high cost, and sensitivity to other plastic additives. Other processing improvements, such as alleviation of surface defects in the extrusion and/or molding of filled thermoplastics, reduction in the pressure-to- fill during injection molding, increase of output rates and reduction of power consumption during extrusion, all without plate-out and exudation of the processing aid, have only met with very limited success and then only in isolated instances.
In contrast, the formulated processing aids of the present invention constitute a new class of versatile processing aids which are broadly useful and very superior to the conventional lubricants and plastic additives of the prior art as will be shown in the following examples.
Thus, there continues a need for an economical solution to the problem which is both satisfactory in terms of processing and yet versatile enough to accommodate a variety of thermoplastic resin systems.